MINI REVIEW article
Sec. Condensed Matter Physics
Low Valence Nickelates: Launching the Nickel Age of Superconductivity
- 1Department of Physics, Arizona State University, Tempe, AZ, United States
- 2Division of Display and Semiconductor Physics, Korea University, Sejong, South Korea
- 3Materials Science Division, Argonne National Laboratory, Lemont, IL, United States
- 4Departamento de Física Aplicada, Facultade de Física, Universidade de Santiago de Compostela, Santiago de Compostela, Spain
- 5Instituto de Materiais (iMATUS), Universidade de Santiago de Compostela, Santiago de Compostela, Spain
- 6Department of Physics and Astronomy, University of California, Davis, Davis, CA, United States
The discovery of superconductivity in thin films (∼10 nm) of infinite-layer hole-doped NdNiO2 has invigorated the field of high temperature superconductivity research, reviving the debate over contrasting views that nickelates that are isostructural with cuprates are either 1) sisters of the high temperature superconductors, or 2) that differences between nickel and copper at equal band filling should be the focus of attention. Each viewpoint has its merits, and each has its limitations, suggesting that such a simple picture must be superseded by a more holistic comparison of the two classes. Several recent studies have begun this generalization, raising a number of questions without suggesting any consensus. In this paper, we organize the findings of the electronic structures of n-layered NiO2 materials (n = 1 to ∞) to outline (ir)regularities and to make comparisons with cuprates, with the hope that important directions of future research will emerge.
After much synthesis and characterization of low-valence layered nickelates over three decades [1–7], superconductivity was finally observed  in hole-doped
FIGURE 1. (A) Crystal structures of the n = ∞, n = 5, and n = 3 layered nickelates highlighting the presence of the n-NiO2 layers and the fluorite blocking slabs present in the n ≠ ∞ materials.
Some overviews on experimental and theoretical findings in this family of materials have been recently published [45–48]. In this paper, we focus on the electronic structure of layered nickelates, confining ourselves to materials with the basic infinite-layer structure: n square planar NiO2 layers each separated by an
2 From ∞ to One
2.1 “Infinite-Layer” n = ∞ Nickelate:
Noteworthy differences from cuprates were already reflected in early electronic structure calculations as well [51, 52]. For the parent material
FIGURE 2. DFT band structures in the paramagnetic state for the n = ∞, n = 5, and n = 3 layered nickelates. The two types of Ni eg bands are highlighted, as well as the two relevant
Besides the presence of
The doped holes tend to be on the Ni sites, as opposed to cuprates where they tend to reside on the oxygen sites. Recent DFT + DMFT Ni 2p3/2 core-level XPS, XAS, and RIXS calculations (consistent with available core-level spectroscopies) indeed confirm that the Ni-O hybridization does not play an important role in connection with doping, implying that the physics of NdNiO2 is well described by a single-band Hubbard model . This in turn brings up the issue of the nature of the doped holes on the Ni sites. That is, do they behave as effective d8 dopants, and if so, is d8 high-spin (S = 1) or low-spin (S = 0)? If the former, then these materials would fall in the category of Hund’s metals, and thus would deviate substantially from cuprates. DMFT calculations are consistent with this picture as they systematically favor high-spin d8 (S = 1) states [40, 57–60]. DFT calculations point instead towards a cuprate-like low-spin (S = 0) picture due to the large crystal-field splitting of the eg states in a square planar environment . Along these lines, impurity calculations show that in the NiO2 layers a Zhang-Rice singlet (like in CuO2) is indeed favored upon hole-doping . Further, cluster calculations find that hole doping distributes over the entire cluster, in contrast to local S = 1 states .
Because of their lower degree of p − d hybridization, the superexchange in
Discussing the origin of superconductivity in
2.2 The Superconducting n = 5 Material
Recently, a second superconducting member has been found in the
In terms of its electronic structure , the n = 5 material is intermediate between cuprate-like and n = ∞-like behavior. From DFT calculations, the charge-transfer energy of Nd6Ni5O12 is ∼ 4.0 eV. This reduced energy compared to the undoped infinite-layer material means that the Ni-3d states are not as close in energy to the Nd-5d states, consistent with the presence of a pre-peak in the oxygen K-edge (similar to what happens with Sr-doped NdNiO2 ). As a consequence, the electron pockets arising from the Nd-5d states are significantly smaller than those in the infinite-layer material (see Figure 2). This reduced pocket size along with the large hole-like contribution from the Ni-3d states is consistent with experiment in that the Hall coefficient remains positive at all temperatures, with a semiconductor-like temperature dependence reminiscent of under- and optimally-doped layered cuprates. Aside from the appearance of these small Nd-derived pockets at the zone corners, the Fermi surface of Nd6Ni5O12 is analogous to that of multilayer cuprates with one electron-like and four hole-like
2.3 The n = 3 Material, the Next Superconducting Member of the Series?
The materials discussed above can be put into the context of earlier studies of bulk reduced RP phases with n = 2, 3 NiO2 layers [5, 74–76], separated by fluorite
The n = 3 member of the series,
The difference between La and Pr trilayer materials could be due to the reduced volume associated with Pr (one of the motivations for the authors of Ref.  to study Sr-doped NdNiO2 rather than Sr-doped LaNiO2). The Ni spin state and metal versus insulator character have indeed been calculated to be sensitive to modest pressure . Another factor is possible mixed valency of Pr as observed in cuprates (though Pr-M edge data on Pr4Ni3O8 did not indicate mixed valent behavior ). Because of its decreased charge-transfer energy relative to n = 5, the rare-earth derived pockets no longer occur  (see Figure 2). This lack of
If superconductivity were to occur, one might hope for a higher Tc as has indeed been predicted via t − J model calculations . Recent RIXS measurements , though, and find a superexchange value for n = 3 nearly the same as that reported for the infinite-layer material. This suggests the possibility that Tc in the whole nickelate family may be confined to relatively low temperatures compared to the cuprates. The similar value of the superexchange for n = ∞ and n = 3 is somewhat of a puzzle. Though their tpd hoppings are very similar, the difference in the charge-transfer energy should have resulted in a larger superexchange for n = 3. The fact that it is not larger is one of the intriguing questions to be resolved in these low valence layered nickelates.
2.4 The n = 2 Material
The n = 2 member of the series, La3Ni2O6, has been synthesized and studied as well [5, 88]. In terms of filling, it lies further away from optimal d-filling, being nominally Ni1.5+: d8.5. Experimentally, it is a semiconductor with no trace of a transition occurring at any temperature, although NMR data suggest that the AFM correlations are similar to those of the n = 3 material. Electronic structure studies  have predicted its ground state to have a charge-ordered pattern with Ni2+ cations in a low-spin state and the Ni1+: d9 cations forming a S = 1/2 checkerboard pattern. This charge-ordering between S = 1/2 Ni1+: d9 and non-magnetic Ni2+: d8 cations is similar to the situation in the n = 3 material . Calculations suggest that it is quite general in these layered nickelates that the Ni2+ cations in this square-planar environment are non-magnetic. This has been shown by ab initio calculations to be the case also with the Ni2+ dopants in the
2.5 The n = 1 Case
Valence counting indicates Ni2+: d8, so a half-filled eg manifold. Conventional expectations are either 1) both 3d holes are in the
While this new nickelate family seems to be emerging as its own class of superconductors, its connections to cuprates (crystal and electronic structures, formal d count in the superconducting region, and AFM correlations) retain a focus on similarities between the two classes. Apart from the obvious structural analogy, the cuprate-motivated prediction of optimal d8.8 filling has been realized in two nickelate materials, one achieved through chemical doping, and the other by layering dimensionality. In this context, the (so far) little studied n = 6 and n = 4 members of the series  may provide some prospect for superconductivity. Oxygen-reduced samples of these materials are so far lacking (though the n = 4 member of the RP series has been epitaxially grown ), and even if they are synthesized, they might require additional chemical tuning to achieve superconductivity. They share a similar electronic structure to the n = 5 material, but with slightly different nominal filling of the 3d bands . Calculations show that as n decreases from n = ∞ to n = 3, the cuprate-like character increases, with the charge-transfer energy decreasing along with the self-doping effect from the rare earth 5d states. In contrast, the particular n = 1 member discussed above seems distinct from other nickelates, and provides a different set of questions in the context of quantum materials [91, 92].
All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.
AB was supported by the United States National Science Foundation, Grant No. DMR 2045826. K-WL was supported by the National Research Foundation of Korea, and Grant No. NRF2019R1A2C1009588. MN was supported by the Materials Sciences and Engineering Division, Basic Energy Sciences, Office of Science, United States Department of Energy. VP acknowledges support from the MINECO of Spain through the project PGC2018-101 334-BC21. WP acknowledges support from United States National Science Foundation, Grant No. DMR 1607139.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Keywords: superconductivity, electronic structure ab-initio calculations, nickelates, magnetism, cuprates electronic structure
Citation: Botana AS, Lee K-W, Norman MR, Pardo V and Pickett WE (2022) Low Valence Nickelates: Launching the Nickel Age of Superconductivity. Front. Phys. 9:813532. doi: 10.3389/fphy.2021.813532
Received: 11 November 2021; Accepted: 14 December 2021;
Published: 02 February 2022.
Edited by:Matthias Eschrig, University of Greifswald, Germany
Reviewed by:Andrzej M. Oles, Jagiellonian University, Poland
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*Correspondence: Antia S. Botana, firstname.lastname@example.org